This project examines the hypothesis that adenosine is a physiological regulator of coronary blood flow. Employing a radioligand binding assay for adenosine, it it proposed to compare cardiac adenosine levels and coronary flow rate to cardiac oxygen usage in open chest dogs to determine whether there is an unique relationship between these variables. Studies in isolated perfused rat hearts and dispersed rat cardiocytes will examine the possibility that cardiocytes release cyclic AMP in proportion to their oxygen need and that this nucleotide is degraded to adenosine by the sequential action of phosphodiesterase and 5'-nucleotidase on the cardiocyte surface.